1. Field of the Invention
The present invention relates to a thermal conductivity detector module and a thermal conductivity detector for a gas chromatograph comprising a heatable resistive detector element configured to be physically arranged in a flow of analytes eluting from a chromatography column and electrically arranged together with resistors in separate arms of a measuring bridge, where the detector element comprises at least two equal detector sub-elements that are configured to be physically arranged in series in the flow of the analytes and electrically arranged in parallel with each other.
2. Description of the Related Art
A thermal conductivity detector of the above mentioned type is known from U.S. Pat. No. 4,080,821 discloses a thermal detector to be used, inter alia, as a flow meter or as a thermal conductivity detector for, e.g., a gas chromatograph. In order to make the detector substantially independent of ambient temperature, the detector has a detector element with three detector sub-elements that have closely matched resistance values and that are arranged in series in the flow of a gas to be measured. Two of the detector sub-elements are electrically arranged in parallel with each other in one arm and the third one in the other arm of the same half of a measuring bridge. An amplifier detects a differential voltage between the connecting nodes of the arms of the respective halves of the measuring bridge and applies an output voltage to the connecting nodes of the halves of the measuring bridge in order to maintain the bridge substantially in balance. This causes heating of the three detector sub-elements, where the heating effect in the third detector sub-element is four times that in the parallel detector sub-elements. If the known thermal detector is used as a thermal conductivity detector, an additional resistor is provided in series with the parallel connected detector sub-elements.
Thermal conductivity detectors are used to detect certain liquid or gaseous substances (fluids) based their characteristic thermal conductivity, particularly in gas chromatography. There, components or substances of a gas mixture are separated by passing a sample of the gas mixture in a carrier gas (mobile phase) through a separation column containing a stationary phase. The different components interact with the stationary phase, which causes each component to elute at a different time that is known as the retention time of the component. The separated substances, also referred to as analytes, are detected by a thermal conductivity detector that has a measuring cell with an appropriate detector element, e.g., an electrically heated filament disposed in a measurement channel. Depending on the thermal conductivity of the analyte flowing past the heated filament, more or less heat is diverted from the heating filament to the wall of the measurement channel, and the heating filament is correspondingly cooled to a greater or lesser degree. As a result of the cooling of the heating filament, its electrical resistance changes, which is detected.
For this purpose and as known from, e.g., U.S. Pat. No. 5,756,878, the heating filament and additional resistors may be disposed in different arms of a measuring bridge. The thermal conductivity of the substance passing the heating filament is obtained from an amount of energy that is supplied to the measuring bridge and is controlled to maintain the temperature of the heating filament at a predetermined temperature. To this end, an operational amplifier detects a differential voltage between the connecting nodes of the arms of the respective halves of the measuring bridge and applies an output voltage to the connecting nodes of the halves of the bridge.
The sensitivity of the detector depends on several factors. Generally, it will be higher, the higher the temperature between the detector element and the wall of the measurement channel is and the higher the resistance of the detector element is. Filaments of metal, in particular gold, have been used for a long time. In order to get a sufficiently high resistance, the filament must be made very thin which, however, leads to poor robustness. Moreover, gases containing hydrogen sulfide can destroy the gold filament. Platinum has some advantages over gold but shows a catalytic effect in gas mixtures that contain hydrogen and hydrocarbons.
A thermal conductivity detector is known from WO 2009/095494 A1, where the electrically heatable filament is micro-machined from doped silicon to achieve a long service life and inertness toward chemically corrosive gas mixtures. Due to its much higher melting point, the silicon filament can operate at a higher temperature than a gold filament. Furthermore, the specific resistance of silicon is higher than that of gold, so that high detection sensitivity is achieved.
A high resistance of the detector element, however, proves to be detrimental if the thermal conductivity detector has to be intrinsically safe. Intrinsic safety (IS) relies on equipment designed so that it is unable to release sufficient energy, by either thermal or electrical means, to cause an ignition of a flammable gas. Thus, intrinsic safety can be achieved by limiting the amount of power available to the electrical equipment in a hazardous area to a level below that which will ignite the gases. There are various IS standards set forth by various certifying agencies for a system to be considered intrinsically safe. Such standards include International Electrical Commission (IEC) IEC 60079-11, Factory Mutual (FM) 3610, Underwriters Laboratories (UL) UL913, Canadian Standards Association CAN/CSA-C22.2 No. 157-92, etc.
The detector element requires a certain electrical power to be heated to and stabilized at the wanted operating temperature. The higher the operating resistance, the higher the voltage across the detector element (P=V2/R, where P, V and R denote the power, voltage and resistance, respectively). The sensitivity of the measuring bridge is maximum if the operating resistance of the detector element and the resistance of a reference resistor in the other arm of the same half of the measuring bridge are equal. The voltage which drives the bridge is then twice the voltage across the detector element. In case of a short circuit of the detector element, the short-circuit current will be limited by the reference resistor and will be twice the operating current through the detector element. Table A.1 of the above mentioned IEC standard, for example, denotes the permitted short-circuit current corresponding to the voltage, where the permitted current decreases highly disproportionately with the voltage increasing. Thus, in view of the relatively high voltage required to drive the bridge with the high-resistance detector element, the reference resistor might not be able to limit the short-circuit current to the permitted value.
This applies in particular, if the thermal conductivity detector is one of a plurality of detectors that are integrated on a thermal conductivity detector module, which module as a whole shall be intrinsically safe. In this case, the individual detectors cannot be treated as separate intrinsically safe devices unless separated by 6 mm through the entire electrical path. Such separation, however, is not feasible when all detectors must be close together as known from modules with four detectors typically used in gas chromatographs. Consequently, the sum of the short-circuit currents of all integrated detectors may not exceed the permitted value.
An approach to solve the problem would be to lower the resistance of the reference, which facilitates a lower voltage to drive the measuring bridge. This would, however, also allow for a greater short-circuit current, thus still violating the IS parameters. Moreover, for the reasons given above, the sensitivity of the measuring bridge would be compromised.